Attorney Charles Cooper said that he found out that his stepdaughter is gay while handling the case for ProtectMarriage.com. Last year, he argued before the Supreme Court to that gay unions could undermine the marriage between a man and woman. The court ruled to strike down Proposition 8, which, backed by California voters, had banned marriage in the state in 2008.

Cooper has since said that his views on gay marriage are evolving. Cooper's stepdaughter Ashley plans to marry her partner in Massachusetts in June. Gay marriage is now legal in 17 states and the District of Columbia.

"My views evolve on issues of this kind the same way as other people's do, and how I view this down the road may not be the way I view it now, or how I viewed it ten years ago," Cooper said in journalist Jo Becker's upcoming book Forcing the Spring: Inside the Fight for Marriage Equality.

Cooper's change of heart reflects a greater trend across the country, according to Ted Olson, one of the lawyers who argued against Cooper in the Supreme Court case. "Every day, people around the country are looking at the marriage issue afresh and challenging their long-held assumptions," Olson said. Most public polls now say that a majority of Americans support allowing gay unions. Cooper told the Associated Press that his family is "typical of families all across America."

"My daughter Ashley's path in life has led her to happiness with a lovely young woman named Casey, and our family and Casey's family are looking forward to celebrating their marriage in just a few weeks," he said.

According to Becker's book, Coooper's family began to look to the plaintiffs in the Supreme Court case, Kris Perry and Sandy Stier, as models for their own daughter. We were so moved to hear of the Cooper family's constant love and support of their own daughter, even as the Perry case was in full swing and Mr. Cooper was spending his days planning Prop 8's defense," Perry and Stier said in a statement to AP. "Some may find this contrast between public and private jarring, but in our opinion, loving an LGBT child unequivocally is the single most important thing any parent can do. We are overjoyed for Ashley and her fiancée, and we wish them the very best."